The 55-year-old crack addict counted his change outside a Harlem liquor store. He had just over a dollar, leaving him 35 cents short of the cheapest mini-bottle.

The 21-year-old heroin addict sat in a McDonald’s on the Lower East Side, wondering when his grandmother would next wire him money. He was homeless, had 84 cents in his pocket and was living out of two canvas bags.

Each was approached by someone who asked the addict for help buying drugs. Using the stranger’s money, each addict went to see a nearby dealer, returned with drugs, handed them over and was promptly arrested on felony drug-dealing charges. The people who had asked for drugs were undercover narcotics officers with the New York Police Department.

A review of the trials in those cases and two others illuminates what appears to be a tactic for small-scale drug prosecutions: An undercover officer, supplying the cash for the deal, asks an addict to go and buy $20 or $40 worth of crack or heroin. When the addict — perhaps hoping for a chance to smoke or inject a pinch — does so, he is arrested.

In the case of the 21-year-old at the McDonald’s, the undercover officer was an unkempt woman who gave the impression she was about to experience withdrawal, the 21-year-old testified. In one of the other cases, an officer allowed an addict to use his cellphone to call a dealer.

It is impossible to determine how widespread this law enforcement tactic is, but the four recent cases reviewed by The New York Times raise troubling questions about the fairness and effectiveness of the way the Police Department uses undercover officers. Officers neither arrested nor pursued the dealers who sold the drugs to the addicts. Instead, the undercover officers waited around the corner or down the block for the addict to return with the drugs before other officers swooped in.

The department’s tactics and prosecutors’ pursuit of such cases have drawn criticism from defense lawyers and juries. In interviews — and, in one instance, in a letter to prosecutors — jurors have questioned why the police and prosecutors would so aggressively pursue troubled addicts. The 21-year-old man and the 55-year-old man were both acquitted of the felony charges.

The tactic would seem at odds with the public positions of some of the city’s top politicians and law enforcement figures, including Mayor Bill de Blasio, a Democrat, and the Manhattan district attorney, Cyrus R. Vance Jr., who have expressed support for reducing prison and jail populations by finding ways to treat mental health problems and addiction.

“We all talk a lot in this city about the public health crisis of drug addiction, and yet we take a very regressive approach to locking people up,” said Tina Luongo, who heads the Legal Aid Society’s criminal practice.

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The McDonald’s on the Lower East Side of Manhattan where Bryan L. said an unkempt-looking woman, who was an undercover officer, asked him to buy drugs for her.CreditMichelle V. Agins/The New York Times

Last year, nearly 5,000 people were charged in New York City with dealing small quantities of heroin or cocaine, and in 2014, just over 6,000 people faced such charges. But the number of those that involved buy-and-bust cases against addicts is unknown. A vast majority of drug-dealing charges end in plea deals, so there are few trials during which such distinctions might emerge.

The 55-year-old crack addict, Reginald J., agreed to speak to a reporter on the condition that only the first letter of his surname be used when identifying him. In an interview, he articulated one of the issues with these sting operations: It is tough for addicts to say no.

“For him to put the money in my hands, as an addict, let me tell you what happens,” he said. “I like to think I could resist it, but I’m way beyond that. My experience has shown me that 1,000 times out of 1,000 times, I will be defeated.”

At one trial in January, a defendant testified that he had shown an undercover officer track marks on his arm. At another trial, in December, the defendant testified that he had even told an undercover officer about his desire to get clean. “You know what? We got to stop getting high,” the man, Mitchell Coward, testified. “That’s what I told him.”

Joan Vollero, a spokeswoman for the Manhattan district attorney’s office, which prosecuted three of the four cases reviewed by The Times, declined to say whether the office considered such sting operations to be appropriate. But she did say that in some cases, addicts who pleaded guilty to felony drug-dealing charges were steered toward treatment instead of prison.

Law enforcement officials said that sting operations remained a necessary and sensible response to neighborhood complaints about drug dealing and narcotics use. For instance, the McDonald’s on the Lower East Side, where the undercover officer approached the 21-year-old heroin addict, was the topic of dozens of recent community complaints about drug dealing and drug use there or outside its doors, the police said.

“They are going to a location where there are prior incidents,” Brian McCarthy, an assistant chief who commands the narcotics division, said in an interview. “And at the same locations, where there are community complaints,” he added.

He acknowledged that the line between users and dealers was not always fixed. “It is common that the people we arrest are also using the narcotics they are selling,” Chief McCarthy said, but he added that his team was after the dealers. “I believe that we attempt to do our jobs in a planned manner with the utmost integrity where we do get people who are selling narcotics.”

Jurors and a judge expressed skepticism in the four cases. One juror, Seth Silverman, wrote a letter to prosecutors after the trial of Mr. Coward in December, saying he felt it was “approaching absurd that you would use the awesome power of your office to represent the people of New York County, along with it and the court’s limited resources, on such a marginal case.”

Since December, juries and judges in Manhattan have acquitted men of the main charge in three of the cases and deadlocked in the trial of a fourth. In each episode, an undercover investigator had approached men, largely at random, at locations where the police believed drug dealing was occurring.

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Reginald J., a 55-year-old crack addict, said it was difficult for drug users to reject offers like the one made to him by an undercover officer. “My experience has shown me that 1,000 times out of 1,000 times, I will be defeated,” he said.CreditKarsten Moran for The New York Times

The 21-year-old heroin addict at the McDonald’s, Bryan L., also agreed to be interviewed on the condition that only the first letter of his surname be used. He described how an anxious, unkempt-looking woman approached the table where he and a friend were chatting. The woman, an undercover officer, would later testify that she approached the table at random.

Bryan L. “was telling me how he was homeless and he didn’t have a place to stay, small talk,” the officer, identified only as No. 279, testified in January.

Bryan L. said that the undercover officer told him she was staying with her grandmother in Brooklyn and was worried she would soon go into withdrawal.

“I said I would help her,” he testified. They walked from the McDonald’s, at Delancey and Essex Streets, toward East Sixth Street, where Bryan L. said he often bought heroin. About a block away, he told the woman and his friend to wait, at the steps of an elementary school. The undercover officer handed him $20. He returned with two bags, which he gave the officer. Minutes later, he was arrested.

He had less than a dollar in change with him and no drugs, a police officer later testified. After the arrest, officers logged the dozens of possessions, including toothpaste, winter hats and stuffed animals, that Bryan L. carried in his two canvas bags.

His lawyer, Sam Roberts of the Legal Aid Society, asked Detective David Guevara, an investigator working on the case, whether any officers of the nine-member field team on the case followed Bryan L. to see where he bought the drugs. The answer was no.

That was a common theme in the three other trials. In one, the addict, who owned no phone himself, had to use an undercover detective’s cellphone to call his drug dealer. But after the addict was arrested, the undercover officer testified he could not remember whether he ever followed up and called the drug dealer’s number, which was logged in his phone, to try to track the dealer down.

The jury took less than an hour to acquit Bryan L. of felony charges of dealing narcotics near a school. Most jurors then remained behind to chat with him after the trial.

One juror said that what troubled the jury the most was that a nine-person narcotics squad — which included two undercover officers, several investigators and supporting officers — would bring a case against a single addict.

“The big underlying question is why a nine-person buy-and-bust team did not follow him to the dealer where he got it from,” the juror, Scott Link, said in an interview. “Everyone was scratching their heads, wondering what the heck is wrong with our system.”

Bryan L. said that even his acquittal had come at a cost. He said he had lost his job at a consignment clothing shop because of the six days he needed to be in court during his trial.

Correction:

Because of an editing error, an article on Tuesday about an apparent New York Police Department tactic in which undercover officers target drug addicts for small-scale drug prosecutions misspelled the given name of a 21-year-old heroin addict who the police said bought drugs near a school for an officer on the Lower East Side. The man, who spoke on the condition that only the first initial of his surname be used, is Bryan L., not Brian. The error was repeated in an accompanying picture caption.

A version of this article appears in print on , on Page A22 of the New York edition with the headline: Officers Snaring Drug Addicts, Not Dealers . Order Reprints | Today’s Paper | Subscribe